Sphere decoding for retransmission diversity in MIMO flat-fading channels

We define an ARQ protocol for MIMO flat-fading channels which varies the bit-to-symbol mapping per retransmission. We begin by defining a model for distinctly mapped transmissions through MIMO channels, and the effect this mapping diversity has on a sphere decoder receiver. Varying the symbol mapping complicates the sphere decoding process, particularly for the enumeration of candidate solutions within the sphere. A technique that promises quick candidate enumeration is suggested, borrowing concepts from existing closest point search schemes. The value of mapping diversity, in reducing BER and reducing computational complexity, is analyzed.